Lead-acid electrochemical cells which are otherwise known as "lead-acid batteries" are commonly used to store and deliver electrical energy. For example, lead-acid electrochemical cells are normally employed in vehicles (e.g. cars, trucks, boats, aircraft, and the like) for ignition, lighting, and other related purposes. These applications are typically known as "SLI" or "starting-lighting-ignition" functions. Lead-acid electrochemical cells are also used in "deep cycle" or traction-related applications.
Conventional lead-acid electrochemical cells include electrically-conductive positive and negative current collectors typically manufactured in the form of foraminous (porous) metallic grids which are manufactured from a lead alloy or elemental lead (99.9%-99.99% purity lead Pb!) as noted in U.S. Pat. No. 3,951,688. The individual current collectors may be planar (flat) in configuration or spirally-wound as discussed further below. Regardless of form, both the positive and negative current collectors (e.g. grids) are supplied with a paste composition that is directly deposited onto both sides of the current collectors during cell production. As a result, positive and negative "plates" are formed from the pasted current collectors. The positive and negative pastes are typically produced from one or more particulate lead-containing compositions which may consist of, for example, finely-divided elemental lead (Pb) or lead compounds (e.g. oxides such as PbO "litharge"! and/or Pb.sub.3 O.sub.4 "red lead"!, as well as lead sulfates PbSO.sub.4 !). The selected lead-containing compositions are then combined with a paste "vehicle" (e.g. water) and various other optional ingredients including sulfuric acid H.sub.2 SO.sub.4 !. Other additives of interest comprise expander materials as discussed in U.S. Pat. No. 4,902,532 which include barium sulfate, carbon black, and lignosulfonate. The expander materials are primarily used in connection with the negative paste as discussed further below.
The paste composition positioned on the positive current collector to form the positive plate in an electrochemical cell is typically characterized as the "positive paste", while the paste composition located on the negative current collector to produce the negative plate is known as the "negative paste". Further information regarding these items and other characteristics of battery paste compositions in general are presented in U.S. Pat. No. 4,648,177 which is incorporated herein by reference. Likewise, methods of applying the paste compositions listed above to the positive and negative current collectors are specifically discussed in U.S. Pat. Nos. 3,894,886; 3,951,688 and 4,050,482 which are also incorporated by reference.
From a functional standpoint, the current collecting members not only collect electrical current within the cell, but likewise provide mechanical support for the paste compositions. The paste compositions specifically function as the active electrochemical materials for storing electrical energy. Battery systems of the type listed above are also commonly referred to as "pasted plate" batteries or "Faure-type" batteries. These particular battery systems (as discussed in U.S. Pat. Nos. 3,894,886; 3,951,688; 4,050,482; and 4,902,532) further include a supply of a selected electrolyte composition therein. Electrolyte materials suitable for this purpose are normally acidic in character, with representative electrolyte compositions including but not limited to aqueous solutions of H.sub.2 SO.sub.4. The electrolyte may also comprise various additives therein such as sodium sulfate Na.sub.2 SO.sub.4 !, phosphoric acid H.sub.3 PO.sub.4 !, and other sulfate salts associated with various counter-ions (e.g. Li.sup.+, K.sup.+, NH.sub.4.sup.+, Mg.sup.+2, and the like). From an operational standpoint, the electrolyte functions as both an ionic current carrier and an active material in both the positive and negative plates.
The selected electrolyte solution may be used in different ways within a given electrochemical cell. For example, the electrolyte may be present in liquid form wherein the electrolyte is not contained or absorbed in any structures. This type of battery is normally characterized as a "flooded battery" or "free electrolyte battery." Flooded batteries are generally constructed from planar (flat) positive and negative plates which are arranged in a parallel configuration having the electrolyte solution therebetween.
In contrast, another type of electrochemical cell which is conventionally known as a "retained electrolyte battery" involves a system in which the electrolyte solution is absorbed and retained within a separator element positioned between the plates. To effectively retain the electrolyte solution within the separator element, the material used to produce the separator element is porous and absorbent. Representative compositions suitable for this purpose include cellulose materials or, more preferably, a "mat" comprised of ultra-fine glass fibers as discussed further below and described in U.S. Pat. No. 4,637,966. Retained electrolyte batteries may involve plate structures which are planar (flat) in configuration and arranged in a parallel orientation having the electrolyte-containing separator member positioned therebetween. Representative electrochemical cells of this type are discussed in U.S. Pat. Nos. 4,421,832 and 5,120,620 (incorporated by reference). In addition, retained electrolyte batteries may also be produced in a spirally-wound configuration in which the positive and negative plates (e.g. the current collectors having the pastes thereon) are wound together with the electrolyte-containing separator element positioned therebetween. Examples of this particular battery type are presented in U.S. Pat. Nos. 4,064,725; 4,212,179; 4,346,151; 4,383,011; 4,606,982; 4,637,966; 4,648,177; 4,780,379; and 5,091,273 which are likewise incorporated herein by reference. Spirally-constructed batteries offer a high degree of efficiency and capacity in a minimal amount of physical space.
Finally, lead-acid electrochemical cells may also be produced in two additional types, namely, (1) sealed; and (2) unsealed. In an unsealed battery system, the interior of the battery case or housing is open to the ambient (outside) environment. Excessive oxygen generated within this type of cell during, for example, overcharging is able to escape from the case. In a sealed battery unit, the housing is hermetically sealed to prevent any communication between the interior of the housing and the outside environment. As a result, oxygen generated during the charging process is consumed internally within the battery. Sealed batteries are also known as "recombinant" or "starved electrolyte" cells. Representative sealed (recombinant) battery systems are discussed, for example, in U.S. Pat. No. 4,383,011.
Regardless of the particular lead-acid battery under consideration, a number of important operating characteristics and capabilities must be carefully considered during the battery design process. One factor of primary concern is "charge capacity". The term basically involves the amount of electricity (e.g. "charge") that can be stored and delivered by the battery. High charge capacity in an electrochemical cell enables the maximum, long-term delivery of electrical energy. Another important factor involves "cycle life". One "charge cycle" is generally defined as the change in electrochemical state of a battery when it goes from a charged condition to an uncharged (e.g. discharged) state. After a certain number of cycles, the charge capacity of the battery begins to diminish.
Finally, the battery reaches a failure point at which it cannot hold and retain a charge, thereby rendering it unusable. It is therefore important for a lead-acid electrochemical cell to have a maximum "cycle life" with this term being defined as the number of cycles which can be experienced by a battery while still maintaining a charge capacity above a useful value typically defined as 50% or 80% of rated capacity.
Battery failure as described above is believed to occur for many different reasons. In flooded electrolyte-type cells, long-term charge cycling typically causes some or all of the paste materials on the plates to separate from the plates and fall to the bottom of the battery case. This process is conventionally known as a "shedding" of the paste. Furthermore, in both flooded and retained electrolyte batteries, charge cycling may cause the paste compositions to chemically deteriorate. This situation contributes to a general decrease in charge capacity leading to eventual failure. Paste deterioration typically occurs for many possible reasons including microscopic morphological changes, formation of inactive lead sulfate, changes in crystalline structure, changes in amorphous character, loss of contact with the current collectors (e.g. grids) and the like. The positive plate in the cell is typically associated with this failure mode.
All of the factors listed above including charge cycle life, charge capacity, and paste stability contribute to the overall effectiveness and useful life of lead-acid electrochemical batteries. These considerations are therefore of primary importance in the development and production of lead-acid energy systems. As discussed further below, the present invention involves a significant advance in battery technology which is characterized by the development of specially-formulated paste compositions that provide improved charge capacity, charge cycle life, paste integrity, and longevity in lead-acid electrochemical cells. Likewise, the pastes of the present invention have a smooth consistency with beneficial rheology characteristics. These features improve the efficiency of the paste application process and also render the pastes more stable during battery operation so that premature paste separation and deterioration is controlled. For these reasons and the other factors discussed below, the present invention represents an important development in the art of electrochemical cell production and design.